1. Field of the Invention
This invention relates to improved catalysts for the selective dehydrogenation of dialkyl aromatic hydrocarbons to produce alkyl vinyl aromatic hydrocarbons, more particularly to catalysts for the production of para-methylstyrene (PMS) via the dehydrogenation of para-ethyltoluene (PET).
2. The Prior Art
The vinyl benzenes play a particularly important role in the preparation of synthetic plastics and resins. The polymerization of styrenes, for example, to produce polystyrene resins is well known.
Styrene and styrene derivatives are typically produced from ethylbenzene materials by dehydrogenation over solid catalysts in the presence of co-fed steam, and at temperatures ranging from 500.degree. C. to 700.degree. C. The catalysts found to be the most effective for this process are those which are based on potassium oxide (carbonate) promoted, chromium oxide stabilized, iron oxide material. Catalysts of this type are said to be self-regenerative inasmuch as, in addition to their effectiveness in promoting dehydrogenation, they also promote the water gas reaction in the presence of the steam co-feed, to thereby remove coke which would otherwise build up on and deactivate the catalyst. The lifetime of such self-regenerative catalysts is thus determined by the effectiveness of the catalyst in maintaining its activity for conversion of ethylbenzene materials such as para-ethyltoluene for any given steam/hydrocarbon ratio in the feed. Catalysts of this type which can maintain such activity at generally lower steam/hydrocarbon ratios are, of course, more economically desirable.
Considerable research has been directed toward attempts to improve the activity and selectivity of iron oxide-potassium carbonate-chromium oxide-based dehydrogenation catalysts. Any improvement which results in either increasing the selectivity (moles of desired product per mole of reactant reacted) or the conversion (moles of reactant reacted per mole of starting material) without lowering the other is economically attractive since the result is that the yield (moles of desired product produced per mole of reactant) of the product has been increased. Any increase in the numerical value of the yield results in a more efficient operation with more reactant being converted into the desired product. In commercial operations, many of which produce millions of pounds of product per year, a trade-off is frequently effected between selectivity and conversion. An increase of only 1 or 2 percentage points in the selectivity can result in a substantial savings of starting materials. An increase in conversion can substantially reduce capital expenditure and energy consumption. The trade-off may vary depending on raw materials costs, energy costs, and the age of the plant.
Attempts have been made to improve the conversion effectiveness and selectivity of iron oxide type dehydrogenation catalysts for use in various alkylaromatic dehydrogenation reactions. Riesser; U.S. Pat. No. 4,152,300; issued May 1, 1979, for example, discloses that an improvement in ethylbenzene dehydrogenation catalyst selectivity can be realized by incorporating small amounts of certain metal oxide materials into dehydrogenation catalyst compositions comprising mixtures of iron oxide, potassium oxide, vanadium oxide and, optionally, chromium oxide.
Courty; U.S. Pat. No. 4,134,858; issued Jan. 19, 1979, discloses an iron oxide based dehydrogenation catalyst containing particular amounts of clay to improve the conversion, selectivity and yield of styrene and divinylbenzenes produced by dehydrogenation of ethyl- or diethylbenzene. This U.S. Pat. No. 4,134,858 also notes that oxides of copper, vanadium, zinc, manganese, magnesium, nickel, cobalt, bismuth, tin and antimony can be added to the disclosed dehydrogenation catalysts.
Notwithstanding such attempts to improve iron oxide based dehydrogenation catalysts, there is a continuing need to formulate catalysts of this type which can be used to realize improved conversion and/or yield in the dehydrogenation of other types of alkylaromatic materials such as, for example, in the production of para-methylstyrene from para-ethyltoluene.
Accordingly, it is an object of the present invention to provide an improved iron oxide based dehydrogenation catalyst especially useful for the dehydrogenation of para-ethyltoluene to produce para-methylstyrene.
It is a further object of the present invention to provide a para-ethyltoluene dehydrogenation process employing a catalyst which provides desirably high conversion of para-ethyltoluene to p-methylstyrene even at relatively low steam to hydrocarbon ratios in the charge to the reaction zone.
These and other objectives can be achieved by means of the invention described and claimed herein.